In recent years there has been active research in the area of nanosecond-type pulse generation. Such research has produced GaAs substrate high power storage devices that utilize photoconductive solid state switching to generate nanosecond-type pulses. One such switch, disclosed in U.S. Pat. No. 5,028,971, issued to Anderson H. Kim et al on Jul. 2, 1991, entitled, "High Power Photoconductor Bulk GaAs Switch" is incorporated herein by reference.
The "Bulk GaAs Switch" is basically an electrical energy storage device comprised of two mutually opposite gridded electrodes positioned on opposite surfaces of a GaAs semiconductor substrate such that a power supply means can provide an electric field in a predetermined direction across the patterned electrodes. The device is photoconductively activated to discharge its stored energy when it receives light radiation at a predetermined wavelength. When light energy penetrates the substrate region it generates electron/hole pairs which cause the electrical resistance of the semiconductor material to decrease. As a result, the stored electrical energy will instantaneously discharge through a load.
It is widely recognized that when such devices discharge properly, they radiate pulses in a direction perpendicular to the substrate surface. The bandwidth of such pulses is determined and/or limited by the speed with which the device discharges and recovers. In general there is a direct correlation between the output pulsewidth and the center frequency of the radiated waveform. As the pulsewidth narrows, the bandwidth increases and the center frequency shifts towards the higher frequency spectrum. For the wider pulsewidth, the center frequency of the radiated waveform becomes lower. Consequently, those skilled in the art recognize that by controlling the output pulse characteristics, one can control the center frequency and bandwidth of the radiated energy, and thus increase the effectiveness as well as the versatility of the systems utilizing such impulse generators.
The critical elements in controlling the radiated pulsewidth of such GaAs energy storage devices are its construction (shape, size, etc.) and the switching technique (photoconductive triggering of the stored energy discharge). Heretofore, two general techniques have been used to generate such narrow pulsewidth (ultra-wideband) radiation.
The first technique utilizes the recombination property of the semiconductor material from which the switch itself is fabricated. This technique (using photoconductive GaAs switches), however, typically generates a signal with a long pulsewidth due to a relatively long recovery time. The long recovery time is attributed to the inherent properties of gallium arsenide, including: (1) the substantially long recombination time and (2) the switch lock-on phenomena. As such, this technique is not desirable for generating narrow pulses and thus for having ultrawideband radiation.
The second technique utilizes an energy storage element which is comprised of either a short section of transmission line or a capacitor. The energy storage element is photoconductively triggered to instantaneously discharge all or substantially most of its stored energy to an impedance load. As with the aforementioned technique, the extended recovery time inherent in photoconductive switches prevents this device from producing extended wideband radiation.
A major breakthrough in this pulsewidth problem, however, was presented in the inventors copending patent application entitled "Ultra-Wideband High Power Photon Triggered Frequency Independent Radiator," Ser. No. 07/946,718, filed by Kim et al, Sep. 18, 1992 now U.S. Pat. No. 5,227,621 and incorporated herein by reference. This frequency radiator combines an energy storage function and an antenna radiating function into one structure to create an ultra wideband frequency radiator capable of generating pulses with a range of frequency components from hundreds of megahertz to several gigahertz. Basically, this radiator utilizes two identical quasi-radial transmission line structures to store electric energy while it implements photoconductive switching to trigger the instantaneous discharge of the stored energy to generate the desired ultra wideband RF radiation.
Such an energy storage device comprises a dielectric storage medium, two quasi-radially shaped, metalized electrodes mounted opposite one another on the top surface of the dielectric storage medium and a metalized electrode mounted on the bottom surface of the dielectric medium. A photoconductive switch, centrally located on the dielectric between the two quasi-radially shaped electrodes, connects the two quasi-radially shaped electrodes to the bottom electrodes through a load impedance. When the switch is activated by light radiation, the stored energy discharges through the load impedance generating a sub-nanosecond-type pulse.
Although such a device provides for fast rise-time pulses, the radiation bandwidth is limited by the trigger speed of the photoconductive switch and the recovery time of the GaAs substrate. One method of breaking through these physical limits and thus increasing the radiation bandwidth is by sharpening the discharge pulse (a sharpened pulse has a fast rise-time as well as fast falltime).
A method of sharpening the radiated pulsewidth has been disclosed in copending patent application, Ser. No. 08/061,612, entitled "Pulse Sharpening Using An Optical Pulse," filed May 6, 1993. The method consists of coupling an optical pulse from a laser system into two separate fibers of different length (Fiber1=L and Fiber2= L+ .delta.L) to create a delay between the signals. Specifically, the optical pulse conveyed through fiber1 triggers the discharge of the device, and the optical pulse conveyed through fiber2 illuminates the gap between the transmission line and the load (to turn off the device). Hence, the energy extraction from the device is abruptly terminated, and the radiating pulse incurs a fast falltime, thus sharpening the pulse.
This pulse sharpening technique, however, is not versatile in that it does not provide the capability for one device to effectively generate a multiple of output pulsewidths having center frequencies ranging from hundreds of megahertz to several gigahertz (which would be useful for many different applications as described above). Consequently, those skilled in the art recognize both the need and the benefit of a device capable of radiating a range of pulsewidths from a single optical pulse source.